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Soil moisture is a critical consideration for agriculture and hydrology especially in semi-arid regions. Like groundwater, soil moisture measurements may not always available in sufficient distributions for effective monitoring of agricultural operations. This paper presents a quasi-terrestrial water balance method, driven by common hydro-climatic data, to estimate soil moisture. The method is tested for the approximate to 152 000 km(2) floodplain region of Hai River Basin using 48 consecutive months (January 2003-December 2006) of data. Comparisons of the estimated and field-measured soil moisture storage and storage change show favorable agreements at monthly, seasonal yearly cycles. This suggests that the proposed method reliably detects soil moisture storage signal in the tested region. The analysis shows a narrowing soil moisture storage change in the study area. Further analysis indicates that the narrowing in soil moisture storage change is sustained by groundwater storage loss via irrigation. There is also storage loss to the atmosphere via irrigation-driven evapotranspiration. Apparently, storage gain occurs in summer while storage loss occurs in winter. However, an overall storage loss exists in the tested floodplain region of Hai River Basin. (C) 2011 Elsevier B.V. All rights reserved.